When a vessel, such as a FPSO—a vessel for floating production, storage and offloading—lies anchored, large forces are required to maintain the position of the vessel. The anchoring lines (the moorings) normally are spread out around a circumference of the vessel. Typically there are 50-100 tons tension in the anchoring lines for this purpose. Pull-in or connection of the anchoring lines is a comprehensive and expensive operation. Normally, the anchors are placed at the seabed in advance, with about half of the length of the anchoring line coupled to each specific anchor. On the vessel the upper half of the anchoring lines are connected, with a pin coupling through adapted lugs or fastening eyes. The locking pins are typically made from high strength steel with a diameter of 200 mm on the pin. The two halves of each anchoring line, of which the lower half is connected to the anchor and the upper half is connected to the vessel, are joined together by use of an installation vessel placed on the surface in position between the vessel and the anchor. For this purpose the upper end of the lower half of the anchoring line and the lower end of the upper half of the anchoring line, respectively, are pulled into the installation vessel, where the joining takes place. To have sufficient length and slack the locking pin in the vessel must be opened, and connected to a winch with a wire or similar the upper half of the anchoring line is fed out from the vessel. After complete joining the anchoring line is retrieved back to the vessel and locked with the locking pin, which is a difficult operation due to the huge tension in the anchoring line. The operation must be repeated for each anchoring line, resulting in significant time and cost spent, not at least because of the requirement of an expensive installation vessel. The locking means has proved to be susceptible to fatigue.
The objective of the present invention is to provide a system and a method for anchoring of a vessel, having distinguishing features making a separate installation vessel unnecessary, and in addition making the pull-in or coupling of the anchoring lines simpler, as well as improving the fatigue properties.